Eye position signals in cat superior colliculus

Effects of eye position upon activity of neurons in macaque superior colliculus

We examined the activity of neurons in the deep layers of the superior colliculus of awake behaving rhesus monkeys during the performance of standard oculomotor tasks as well as during self-guided eye movements made while viewing natural images. The standard tasks were used to characterize the activity of neurons based on established criteria. The natural viewing paradigm enabled the sampling of neuronal activity during saccades and fixations distributed over a wide range of eye positions. Two distinct aspects of eye-movement behavior contributed to the modulation of firing activity in these neurons. The well-established influence of saccade amplitude and direction was strongest and most prevalent surrounding the time of the start of the saccade. However, the activity of these neurons was also affected by the orbital position of the eyes, and this effect was best observed during intervals of fixation. Many neurons were sensitive to both parameters, and the directions of their saccade vector and eye position response fields tended to be aligned. The sample of neurons included visual, build-up, and burst activities, alone or in combination. All of these activity types were included in the subpopulation of neurons with significant eye-position tuning, although position tuning was more common in neurons with build-up or burst activity and less common in neurons with visual activity. The presence of both eye-position as well as saccade-vector signals in the superior colliculus is likely important for its role in the planning and guidance of combined movements of the eyes and head.

Gaze field properties of eye position neurones in areas MST and 7a of the macaque monkey

The activity of parietal cortex neurones primarily related to eye position (EP neurones) was studied in macaque monkeys with the aim of precisely defining the neurones' gaze fields (GF) and comparing them in two functionally different areas, MSTd and 7a. Discharge rates of single neurones in the inferior parietal lobule and in the underlying cortex of the superior temporal sulcus were recorded in two Java monkeys while the animals fixated a steady visual target positioned at several different points on a video screen. The GFs were then drawn as a regression surface fitting the mean discharge rates. Cells tonically influenced by the angle of gaze were found in both areas. The GFs most often took the form of a nearly planar surface best characterized as a ramp tilted towards a hemifield or quadrant of the visual field, shifted eccentrically with respect to the straight ahead (primary position), and with a midpoint centred between 0 deg and 20 deg of gaze eccentricity and saturation between 10 deg and 35 deg. In a minority of cases, the discharge rate was nearly maximal at the primary position and decreased to a minimum within 35 deg of eccentricity. In other instances, the GFs were peaked surfaces, limited to a restricted part of visual space. EP neurones, while showing similar gaze fields in areas MST and 7a, were found intermingled with functionally different types of cells. The results suggest that EP neurones similar to those already described in several areas of the monkey parietal cortex are present also in area MST. These cells, by signalling the degree of gaze eccentricity from the primary position, encode gaze position in an orbito-centered frame extending up to 30-35 deg from the straight-ahead. The role of EP neurones might be to supply contiguous elements with a gaze eccentricity signal required for visuo-motor processes such as the control of tracking movements.

Ibotenic acid lesions of the lateral substantia nigra restore visual orientation behavior in the hemianopic cat

Transection of non-tectotectal fibers in the caudal one-half of the commissure of the superior colliculus restores visual orienting to a cat previously rendered hemianopic by a large unilateral visual cortical lesion. Other observations related to this recovery phenomenon (i.e., the Sprague effect) have suggested that the caudal commissural fibers whose destruction produces the recovery 1) are contralateral afferents to the superior colliculus on the side of the cortical lesion, and 2) profoundly influence visuo-motor processing in this superior colliculus. We performed anatomical and behavioral experiments to determine which of the more than 40 contralateral collicular afferents are directly involved in the Sprague effect. To guide subsequent behavioral studies, we performed a pilot anatomical experiment in which we injected WGA-HRP unilaterally into one superior colliculus at identical retinotopic loci in each of a pair of cats. One cat was normal (control), and the other (experimental) had previously received a caudal transection of the collicular commissure. Quantitative comparison of the retrograde labeling in collicular afferents revealed that a number of mesencephalic regions contain neurons that project to the colliculus via the caudal collicular commissure. Additional collicular injections of WGA-HRP demonstrated the exact location and distribution of collicular afferent neurons within these nuclei. In the behavioral experiments, we attempted to replicate the Sprague effect by destroying the neurons giving rise to the axons in the caudal collicular commissure. Ibotenic acid lesions of these neurons were performed in cats that were hemianopic following the removal of the contralateral visual cortex. Small lesions of a "critical zone" in the rostro-lateral substantia nigra pars reticulata and possibly the overlying ventral zona incerta consistently produced a visual recovery whereas lesions of the other collicular afferents did not. Paradoxically, large nigral lesions that also included the critical zone did not result in a recovery. A conceptual framework for these findings involving striato-nigro-tecto-preoculomotor interactions is presented.

Neuronal activity related to head and eye movements in cat superior colliculus

1. Movement-related discharges were recorded from single cells in the superior colliculus of alert cats while they made eye saccades (with head fixed) or gaze saccades (with head free). 2. Visual and auditory stimuli were used as saccade targets. In addition, saccades were made to the remembered location of targets and spontaneously, in the absence of targets, during intertrial intervals. 3. When the head was still and the cat performed either spontaneous saccades or saccades to remembered targets, only one class of tectal neurone, the saccade-related burst neurone, inevitably discharged prior to all saccades of appropriate amplitude and direction. 4. Neurones with longer lead times and less intense presaccadic discharges were obligately linked only to visually elicited saccades. The discharge of some of these long-lead neurones was also influenced by the spatial position of the visual target (that is, by craniotopic motor error), while that of the saccade-related burst neurones was a function of retinocentric motor error. 5. Most neurones which discharged before head movements also discharged before eye movements and had large, contralateral movement fields.

The perihypoglossal projection to the superior colliculus in the rhesus monkey

The projection of the perihypoglossal (PH) complex to the superior colliculus (SC) in the rhesus monkey was investigated using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Following physiological identification by electrical stimulation and multiunit recording, small injections of the tracer were placed within the SC of three monkeys. The largest numbers of retrogradely labeled neurons within the PH complex were found in the contralateral nucleus prepositus hypoglossi (NPH), in the laterally adjacent medial vestibular nucleus, and in the ventrally adjacent reticular formation (the nucleus reticularis supragigantocellularis). These labeled neurons are strikingly heterogeneous in size and morphology. The nuclei supragenualis and intercalatus also contain numerous labeled neurons in the 2 cases in which the injections involve the caudal SC. Large numbers of retrogradely labeled neurons as well as anterogradely transported WGA-HRP are observed also throughout the pontine and medullary reticular formation, including the midline raphe. The PH complex, particularly the NPH, is known to be involved in the coding of eye position and has been hypothesized to be a critical component of the "neural integrator." Our data demonstrate the existence of a robust projection from the PH complex to the contralateral SC in the rhesus monkey. This projection may serve as the anatomical substrate by which a corollary of eye position could reach the SC. Such a signal is a prerequisite for the computation, at the collicular level, of saccadic motor error signals observed in the SC of rhesus monkeys.

Visual responses of neurones in cat superior colliculus in relation to fixation of targets

1. Visually elicited discharges were recorded from single cells in the intermediate and deep layers of the superior colliculus of alert, trained cats. With the behavioural protocols used, vigorous and consistent visual responses to the onset and offset of small, stationary targets were encountered in all layers of the colliculus. In addition, the responses of many cells depended on the behavioural context in which the response occurred. Specifically, thirty-nine cells were encountered in which discharge was contingent upon prolonged fixation of the visual stimulus. Thirty of these were located in the intermediate and deep layers and comprised 22% of the 134 isolated cells which were not related to saccadic eye movement. 2. Fixation-related cells showed sustained increases or decreases in firing during fixation of a visual target. Responses to presentation of a visual target at the same retinal location, without active fixation by the animal, were either absent, much weaker, or had extremely variable latencies. 3. Changes in activity were well established by the end of the fixation saccade. The neuronal response was sustained if fixation was sustained. Moreover, the neuronal response began to decrease before the onset of the saccade which broke fixation even though the receptive field location of the target had not yet changed. 4. In these neurones, the discharge rate was not a function of the position of the eye in the orbit. 5. Neurones in which the discharge rate was suppressed during fixation had larger receptive fields than cells in which the discharge rate was enhanced during fixation. There were fewer fixation-suppressed cells (n = 7, 5%) than fixation-activated cells (n = 23, 17%).